There is much information available on the production of trimethylene carbonate. Most of the prior art describes processes which go through poly(trimethylene carbonate) which is most commonly made from 1,3-propanediol and a carbonate source. The carbonate source can be dialkyl carbonates, alkylene carbonates, phosgene, urea, etc. U.S. Pat. No. 5,212,321 describes a process for producing trimethylene carbonate wherein 1,3-propanediol is reacted with diethyl carbonate in the presence of zinc powder. U.S. Pat. No. 6,054,596 describes producing cyclic carbonic esters by reacting a diol with a carbonic ester using a salt of a weak acid with an alkali metal or an alkaline earth metal as a catalyst.
All of this prior art utilizes batch reactive distillation of the poly(trimethylene carbonate) (PTMC) to trimethylene carbonate (TMC). However, reactive distillation of TMC from PTMC results in poor yields when done in a conventional batch distillation apparatus. The yields decrease with increasing batch size. The relatively high temperatures under which this batch reactive distillation takes place, i.e., from about 180 to about 230° C., and the relatively long length of time during which the PTMC and TMC are exposed to this temperature are the causes of a disadvantageous heat history for this distillation. This causes undesirable side reactions which can include dehydration of PTMC endgroups, further reactions of the dehydrated endgroups, decarboxylation of PTMC carbonate groups, and thermal degradation of the decarboxylated groups which result in poor yields of TMC, i.e., only from about 20 to about 70 percent, and most commonly for larger batches, less than 50 percent. Thus it can be seen that it would be advantageous to provide a continuous process which minimizes the heat history of the PTMC during the reaction to TMC.